1. Field of Invention
This invention relates to a method and apparatus for automatically calibrating cameras used for scanning documents as part of an imaging process.
2. Background Information
Document scanners are used to capture an image of a document as the document and the document scanner or "camera" are moved relative to each other. A typical document scanner scans a document to generate various levels of gray which are then thresholded into black and white pixels, for example. The resulting black and white pixels are then used to represent the image of the document.
Some of the problems with document scanners are that the scanners tend to age and collect dust which tend to distort the gray scale values generated by the scanners. As a result, the scanners need to be calibrated to correct the problems mentioned.
Two general methods for calibrating document scanners are as follows:
(1) Use a permanent white reference background or member which is mounted along a document track to be viewed by the scanner; and PA1 (2) Use a white reference card which is passed in front of the scanner during the calibration process. PA1 (1) When a stationary white reference member is used, it is difficult to detect the leading edge of a document as the document is moved towards the optical axis of the document scanner, and it is also difficult to detect the height of the document against the background of the white reference member. With the passage of time, the white reference member becomes contaminated through the constant wear of documents passing thereover; and PA1 (2) Using a white reference card or document being fed in the document track relies on human intervention to calibrate the camera or document scanner and is subject to the experience level of the operator performing the calibration. PA1 a moveable member moveable between first and second positions relative to an optical axis of an imaging device positioned at a document track; PA1 said moveable member having a first reference member positioned relative to said optical axis when said moveable member is positioned in said first position, and said moveable member having a second reference member positioned relative to said optical axis when said moveable member is positioned in said second position; and PA1 an actuator mechanism for moving said moveable member between said first and second positions. PA1 (a) providing a moveable member having said black and white reference members thereon; and PA1 (b) moving said moveable member to position said white reference member and said black reference member with regard to said optical axis as required by a controller calibrating said optical device.
There are certain limitations which are inherent in the two approaches mentioned: